In general, a color cathode ray tube comprises a vacuum envelope having a face panel, a phosphor screen formed on an inner surface of the face panel and including three color phosphor layers capable of radiating in blue, green, and red, a shadow mask opposed to the phosphor screen, and an electron gun provided in a neck of the vacuum envelope. The shadow mask includes a mask body having a number of apertures for passing electron beams, and a mask frame supporting the peripheral edge portion of the mask body. In this color cathode ray tube, three electron beams emitted from the electron gun scan the phosphor screen through the shadow mask, thereby displaying a color image.
The shadow mask is provided to select the three electron beams to be respectively landed on predetermined positions on the three color phosphor layers, and this selection must be correctly carried out such that three electron beams are respectively landed correctly on predetermined positions of the three color phosphor layers, in order that a color image displayed on the phosphor screen obtains an excellent color purity. Therefore, the shadow mask must be arranged so that a predetermined positional relationship is always maintained with respect to the phosphor screen during operation of the color cathode ray tube, i.e., the distance (q value) between the shadow mask and the phosphor screen must always fall within a predetermined tolerance range.
However, in a color cathode ray tube of a shadow mask type, only 1/3 or less of the entire electron beams emitted from the electron gun reach the phosphor screen, and the other remaining beams collide onto the shadow mask. Further, the shadow mask is heated by those colliding electron beams and expands towards the phosphor screen, i.e., so-called doming occurs. The doming can be divided into two types.
One type that occurs is at the beginning of starting operation of a color cathode ray tube. Specifically, at the starting operation, the mask body of the shadow mask is mainly heated and a temperature difference occurs between the mask body and the mask frame which is provided on the peripheral edge portion of the mask body. Due to the temperature difference, doming occurs.
The other type that occurs is locally in a relatively short time when an image having a high luminance is locally displayed and the mask body is thereby locally heated and expanded.
Once doming of a shadow mask occurred, the position of the shadow mask relative to the phosphor screen changes and the q value derives from the tolerance range. Landing positions of electron beams with respect to the phosphor layers are then dislocated from predetermined positions, and as a result, the color purity of an image displayed is degraded. Landing dislocations thus caused by doming vary depending on the position of an image pattern to be displayed, the luminance thereof, and the continuation time of a high-luminance image pattern.
In addition, a landing dislocation of an electron beam caused by local doming when an image having a high luminance is displayed locally tends to easily occur at an intermediate region between the center of the shadow mask and an end of the horizontal axis thereof. This can be associated with doming of the shadow mask and the deflection angle of an electron beam. For example, even when doming occurs in the vicinity of the vertical axis of a shadow mask, the deflection angle of electron beams is small within this portion, so that the electron beam is not much affected by doming and a landing dislocation caused therefrom is small. Meanwhile, the peripheral portion of the mask body is supported on the mask frame which has a large heat capacitance by a non-aperture portion, so that heat in the mask body diffuses into the mask frame even when the peripheral portion of the mask body is locally heated. Therefore, doming which occurs in the peripheral portion of the mask body is of a low level and causes only a small landing dislocation.
In contrast, in an intermediate region between the center of the shadow mask and each end of the horizontal axis thereof, electron beams have a large deflection angle, and doming of a high level occurs when the shadow mask is locally heated within these intermediate regions. As a result, a landing dislocation tends to occur most easily at those portions of the phosphor layer which face the intermediate regions of the shadow mask.
In order to prevent a local heat expansion of a shadow mask and to prevent color blurring, the curvature of a shadow mask in its horizontal cross-section should be enlarged. In recent years, however, it has been a main trend to use a color cathode ray tube having a flattened face panel, and accordingly, such a cathode ray tube has a flattened shadow mask. Therefore, it is difficult to restrict local doming which occurs in a relatively short time and to eliminate a landing dislocation, only by means of enlarging the curvature of the shadow mask in its horizontal cross-section.
In a television set incorporating a color cathode ray tube, a landing dislocation occurs when a vibration caused by sounds or voices from a laud speaker during operation of the television set is transferred to the color cathode ray tube, the mask body itself vibrates (or causes howling) and causes a landing dislocation of electron beams, in addition to a landing dislocation caused due to doming of the shadow mask as described above. Therefore, such a landing dislocation caused by howling must be restricted.
Since the peripheral edge portion of a shadow body is fixed to a mask frame, a vibration has a small amplitude in this portion. However, in the intermediate regions of the mask body as described above, the vibration is large and a landing dislocation has the largest amount.